When certain high intensity discharge (HID) lamps are operated in a vertical orientation, the lamps display a phenomenon called vertical segregation. Vertical segregation is identified by a non-uniform color of light emitted from the discharge along the vertical axis. More specifically, the color of the HID lamp varies from the top to the bottom. Vertical segregation is caused by gravity creating a non-uniform distribution of chemistry in the lamp. Simply put, a majority of the chemistry is located at the bottom of the arc rather than being evenly distributed throughout the arc.
In one attempt to overcome the disadvantage of vertical segregation, the second longitudinal acoustic mode was excited. When the second longitudinal acoustic mode was excited, the chemistry in the lamp was mixed, which yielded a uniform color along the vertical axis of the HID lamp. In other words, the vertical segregation was reduced. Furthermore, it has been demonstrated that driving the HID lamp at a high frequency which was swept from 45 Hz to 55 kHz at a slow rate (for example, 100 Hz) provided arc stability and arc straightening.
In another attempt, a full bridge configured resonant inverter was operated at a high frequency (for example, 250 kHz) in order to synthesize an amplitude modulated 45 kHz to 55 kHz sinusoidal waveform through the use of pulse width modulation (PWM) techniques. The amplitude modulated waveform generated the second longitudinal acoustic mode excitation used for color mixing or reducing vertical segregation. The required high frequency switching and the PWM operation makes the implementation inefficient since the circuit power losses are very high.